1. Technical Field
The present disclosure relates to a microelectromechanical structure having enhanced mechanical characteristics for rejection of acceleration noise, in particular, the following discussion will reference, without implying any loss of generality, to a gyroscope of a microelectromechanical type.
2. Description of the Related Art
Micromachining techniques enable manufacturing of microelectromechanical structures or systems (MEMS) within layers of semiconductor material, which have been deposited (for example, a layer of polycrystalline silicon) or grown (for example, an epitaxial layer) on top of sacrificial layers, which are removed via chemical etching. Inertial sensors, accelerometers, and gyroscopes made with this technology are experiencing a growing success, for example, in the automotive field, in the inertial-navigation sector, or in the sector of portable devices.
In particular, integrated gyroscopes made of semiconductor material using MEMS technology are known. These gyroscopes operate on the basis of the theorem of relative accelerations, exploiting the Coriolis acceleration. When an angular velocity is applied to a mobile mass that is driven with a linear velocity, the mobile mass “feels” an apparent force, called Coriolis' force, which determines a displacement thereof in a direction perpendicular to the direction of the linear velocity and to the axis about which the angular velocity is applied. The mobile mass is supported via springs that enable a displacement thereof in the direction of the apparent force. On the basis of Hooke's law, the displacement is proportional to the apparent force in such a way that from the displacement of the mobile mass it is possible to detect the Coriolis' force and a value of the angular velocity that has generated it. The displacement of the mobile mass can, for example, be detected in a capacitive way, determining, in conditions of resonance, the variations of capacity caused by the movement of mobile electrodes, fixed with respect to the mobile mass and coupled to fixed electrodes.
MEMS gyroscopes generally have symmetrical sensing structures, comprising a pair of sensing masses for each axis of detection about which a corresponding angular velocity is detected. Ideally, an altogether symmetrical structure enables rejecting completely, by means of the use of appropriate differential reading schemes, linear noise accelerations that are applied from the outside, for example, which can be imputed to shock acting on the sensor or to the gravity acceleration. In fact, whereas the Coriolis' force tends to unbalance in opposite directions, and substantially by the same amount, the sensing masses of each pair (generating movements “in phase-opposition”), the external noise accelerations determine displacements in the same direction and again by the same amount (generating movements “in phase”). By executing the difference of the electrical signals associated to the two sensing masses of each pair, it is possible to measure the contribution due to the Coriolis' force and reject completely the noise contributions of the accelerations.
The inevitable spread of the manufacturing process, and in particular the resulting differences, even minimal, in the mechanical characteristics of the sensing masses and of the corresponding elastic supporting elements, are such that gyroscopes of a traditional type are not perfectly immune from acceleration noise coming from outside.
In fact, even though the vibration modes of the sensing masses are uncoupled and ideally at the same frequency, due to process spreads, the resonance frequencies of the two sensing masses of each pair cannot be perfectly coincident. For example, they can differ by 10-20 Hz, which causes, for high factors of merit Q, a poor rejection to the external acceleration noise. In particular, external accelerations having a frequency close to the frequencies of resonance of the sensing masses can generate responses even considerably different in the two sensing masses, thus generating a non-zero output from the corresponding reading electronics (notwithstanding the differential scheme adopted is ideally able to reject the noise). Considering that the resonance frequency of the sensing masses is usually comprised in the audio band (i.e., less than 20 kHz), it is evident that environmental noise can also generate, for the reason set forth above, even relevant noise at output.